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Comment by xenadu02 | Hacker News Reader

There's a lot of water in the way between DNA and any microwave source. Microwaves do not penetrate water very well - part of the reason rain tends to cause problems with microwave links. Also the reason microwave ovens work: by heating the food.

So what is the proposed mechanism by which microwave energy is supposed to heat DNA enough to damage it?

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Microwave ovens are able to heat several pounds of meat all the way through to the center. So it can penetrate at least a few wet inches of tissue.

In my experience they heat the outside but the center warms by conduction.

If I heat a bowl of tomato sauce it can be boiling at the edges and cold in the middle. I have to stop and stir it a few times.

Same with potatoes. Hot on the outside, raw in the center.

So I went poking around on the Intarwebs for "microwave penetration" and found this rather intriguing page with penetration depths of various items: http://www.pueschner.com/en/microwave-technology/penetration...

I was surprised by the table. Temperature matters way more than I'd expect for water, and it seems ice may in fact be effectively transparent to microwaves? Now I want to fiddle when I get home... it looks an awful lot like you can put a cup of water and a chunk of ice in a microwave and boil the water while the ice is still frozen... (though I'm going to assume that number for ice is pure ice; air bubbles in ice may wreck this up) Experimentation time!

Someone came up with a recipe exploiting the fact that microwaves are absorbed much more readily by fluids than solids. It was a frozen meringue with a hot center. I think it was in Sci Am. The name suggested the reverse of a Baked Alaska.

Now that's some clever cooking. I'd not have thought of that.

That's thermal conductivity, not penetration.

About the only thing that can be reasonably uniformly heated in a microwave at continuous full power is a water-based liquid and that only because of convection.

Put a slab of meat in for a minute at 100% power and it'll be still cold in the center.

> So it can penetrate at least a few wet inches of tissue.

As anyone who's used a microwave a few times in their life will tell you, microwaves, at best, manage to super-heat a fraction of an inch of food.

And that's from blasting >1,000 watts of radiation at it.

Microwave ovens output something like 700-1000W in a shielded chamber continuously over the course of a minute or longer.

Even then they’re generally only heating the outer layers, which in turn heat the inner layers through thermal conduction.

No, a microwave oven penetrates meat half an inch max.

Most of the microwave power does not deeply penetrate something like a whole ham. The microwaves are mostly absorbed by the outer layers and then the heat conducts through the inner portions.

The journal article below contains lots of detail. It claims that microwaves penetrate about 3.8 mm in cooked ham and 9.9 mm in cooked beef.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4657497/

> The microwaves are mostly absorbed by the > outer layers and then the heat conducts > through the inner portions.

Then why does the plate have to be slowly rotated within the oven? I used to believe that it was for the waves peaks (distant from one another by a pair of centimeters) to be able to reach every point of the inside.

But if the inside is heated by conduction, which propagates evenly in all directions, then why care rotating the plate?

Microwave ovens do indeed have hot spots[0]. Conduction doesn't propagate evenly in very many foodstuffs, and you still want to avoid overheating any one spot.

  [0] https://www.evilmadscientist.com/2011/microwave-oven-diagnostics-with-indian-snack-food/

Sure, when you use hundreds of watts over several minutes.

The decay over distance should be roughly exponential: every x millimeters, you're left with only half of the radiation. Given the radiation caused by a cellphone in your pocket, how many microns of water do you need to attenuate the intensity to a non-mutagenic level?

Microwave ovens also typically transmit around 1kW, at the resonant frequency of water.

By comparison most cell equipment is either not at this frequency or a harmonic if it, limited to <<50W, or both.

What do you mean by there's a lot of water between DNA and microwave source? Outer skin cell have a diameter of 30 microns - their DNA can be damaged.

Don't microwaves work by literally boiling the water in food, from the inside out?

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